The crystal structure of plate-like Co nanoparticles can be controlled by a simple solvothermal process in which ethylene glycol was used as both solvent and reducing agent and cetyltrimethylammonium bromide (CTAB) as control agent. Single HCP phase, single FCC phase, and HCP and FCC mixed Co plate-like nanoparticles were synthesized. It was found that the crystal structure of the plate-like Co nanoparticles can be tuned by controlling the CTAB concentration and the reaction time. The FCC-to-HCP phase transformation in the plate-like Co nanoparticles can be promoted, delayed, and prevented by increasing the CTAB concentration from 0 to 50 mM for a reaction time of 15 h. The plate-like Co nanoparticles synthesized with the CTAB concentrations between 0 and 50 mM for the reaction time of 5 h are the single FCC phase. The fraction of the HCP phase in the HCP and FCC mixed Co plate-like nanoparticles increases with the reaction time for the CTAB concentration less than or equal to 9.5 mM. For 5 mM CTAB in the reaction solution at 250°C, as the reaction time increases from 5 h through 12 h to 15 h, the plate-like Co nanoparticles transform from the single FCC phase through the mixture of the HCP and FCC phases to the single HCP phase. Magnetic measurements reveal that both single-phase HCP and FCC Co nanoparticles are ferromagnetic. The crystal structures of the plate-like Co nanoparticles have a strong effect on their magnetic properties. Compared with the FCC Co plate-like nanoparticles, the HCP Co plate-like nanoparticles show a higher coercivity.